Micromeritics Part 2

Question 1

Outline the strengths and limitations of optimal microscopy

Answer 1

Pros
- Visualisation (can see the particles)
- simple, quick and inexpensive qualitative analysis
Cons
- shows 2D image of particles
- doesn’t show particle surface details
- Overlapping particles or agglomerate mistaken as a large single particle
- Optical resolution: 1.2 microns , lower limit 0.2 – 0.5 micrometre
- Need good contrast between particles and background
- Transmission (limited surface information)
- Tedious for quantitative analysis

Question 2

Outline the strengths and limitations scanning electron microscopy

Answer 2

Pros:
- higher resolution
- can see particle surface details
- lower limit 1nm
Cons:
- 2D
- Overlapping particles or agglomerate mistaken as a large single particle
- Need good contrast between particles and background

Question 3

Outline the strengths and limitations of Sieving

Answer 3

Pros
- Simple, quick and inexpensive quantitative analysis
- Wide particle size range
Cons:
- Lower size limit 25 – 50 micrometre
– Low resolution (several tens of micrometre)
– Brittle materials can fracture
– Particle orientation
– Labour-intensive

Question 4

Outline the strengths and limitations of the Andreasen apparatus

Answer 4

Pros:
- A direct measure of the Stokes diameter
– Easy & inexpensive
– Simple to operate (but tedious)
Cons
-Lower size limit ~1 micrometre
– Particle interactions: < 2% (w/v) concentration

Question 5

What is the principle for Laser Diffraction?

Answer 5

light diffraction occurs at the surface of a particle which has
a refractive index different from its surrounding

• scattered light intensity increases with increasing particle
  size
• scattered light angle increases with decreasing particle size

Question 6

Outline the strengths and limitations of Laser Diffraction

Answer 6

Pros:
- Simple to use, quick (few minutes) to measure equivalent volume diameter
– Wide particle size range (0.01 - 3500 μm) and higher resolution
– Both wet and dry samples can be used.

Cons
- Dense suspensions or aerosols – secondary scattering
– Refractive index
– Particle shape
– Expensive instrument

Question 7

How does the Coulter Counter Work?

Answer 7

A glass test tube (has an orifice) is put inside a beaker containing an electrolyte solution. Inside the tube there is a positve electrode and outside the tube there is a negative electrode. When electrode is connected to a power source = generation of electric current and will flow from negative to positive end (inside the tube
When you add the powder - particles will go throught the orifice -….
- can work out the size of particle

Question 8

Outline the strengths and limitations of Coulter Counter

Answer 8

Pros
- A direct volume-base diameter measurement
– Rapid, measuring a few thousands of particles per second
– Lower size limit ~0.5 micrometres. Particles around this size will be difficult to work with if this process is used. If size is more than 1/2 a micron or 1 micron than its suitable

Cons
- Conductive liquid is required
- Needs to make sure particle wont dissolve in conducting liquid
– Coincidence error. If you have 2 particles going through the oriface at the same time machine cannot differentiate whether it is a large single particle going through or 2 smaller particles going through
– Orifice clogging